I just purchased an anemometer so I can calculate flow into returns and out supplies to see if there are any serious duct leaks. Once I know the average velocity through the louvers, I need to plug in an area for the calculation.
Seems to me there is a relatively constant factor, (perhaps mfgr. dependant) somewhere in the vicinity of .75-.81 to use, but I am not sure of:
1. what factor to use?
2. where to measure the louver: outside edges of frame, or inside where the louvers begin?